Electrical insulating oils are sometimes formulated by adding synthetic oxidation inhibitors such as di-t-butyl-p-cresol or di-t-butyl phenol. To meet certification requirements in some jurisdictions, synthetic oxidation inhibitors can not be employed. Oils formulated without such synthetic inhibitors are sometimes referred to as “uninhibited” electrical oils. References relating to synthetically inhibited or uninhibited electrical insulating oils include U.S. Pat. Nos. 3,932,267 (Lewis et al.), U.S. Pat. No. 4,018,666 (Reid et al.), U.S. Pat. No. 4,062,791 (Masunaga et al. '791), U.S. Pat. No. 4,070,297 (Masunaga et al. '297), U.S. Pat. No. 4,082,866 (Link), U.S. Pat. No. 4,124,489 (Reid), U.S. Pat. No. 4,125,479 (Chesluk et al.), U.S. Pat. No. 4,170,543 (Lipscomb, II et al.), U.S. Pat. No. 4,240,917 (Pearce, Jr. et al.), U.S. Pat. No. 4,542,246 (Matsunaga et al. '246), U.S. Pat. No. 4,846,962 (Yao), U.S. Pat. No. 6,355,850 B1 (Angelo et al.), U.S. Pat. No. 6,689,872 B2 (Kent et al.) and U.S. Pat. No. 6,790,386 B2 (Fefer et al.), and SHELL DIALA® OILS A & AX Electrical insulating oil (product literature from Shell Oil Co.)